1. Field of the Invention
The present invention relates to document image acquisition, and particularly to ensuring that the acquired image data will be of high quality and a resolution suitable for the content of the image, even if the image contains text together with halftone (grayscale levels) or color image, or both.
2. Description of the Related Art
As increasingly larger storage devices have become available, it has become possible to store a document not simply as ASCII text but also as a full facsimile image of the document. More specifically, it is now commonplace to convert a document into a computer-readable bit map image of the document and to store the bit map image. Accordingly, whereas ASCII text storage permitted storage and display of only text portions of documents, it is now possible to store a document in computer readable form and to display not only the text but also pictures, line art, graphs, tables and other non-text objects in the document, as well as to show the text in the actual font and style used in the original document. Likewise, it is possible to store and display documents such that text attributes, such as size, position, etc., are preserved.
FIG. 3 shows a page of a representative document. In FIG. 3, a document page 40 is arranged in a two-column format. The page includes title blocks 41, 42, 47 which include text information of large font size suitable for titles, text blocks 43, 44, 48, which include lines of text data, graphics blocks 45, 46 which include graphic images which are not text (in this example, they are a line drawing and a full-color image), a table block 49 which includes a table of text or numerical information, and a caption block 50 which includes small text data and which is a caption associated with blocks of graphic or tabular information.
Despite the technical advances mentioned above, however, it is still difficult to store document images in computer memory efficiently, because of the large amount of information required for even one page. For example, at 300 dots-per-inch resolution, an ordinary 81/2 by 11 inch black and white document requires approximately 8.4 million bits to store a full document image (assuming that only one bit is used per dot, which is possible with monochrome text and line drawings, but not with images containing grayscale image or color image portions). Adding grayscale image or color to the document, or increasing the resolution at which the image is stored, can easily increase storage requirements to many tens of millions of bits per page. Moreover, the time required to retrieve those bits from storage and to create and display the resulting image is significant, even with current high speed computing equipment. The time is lengthened even further in situations where the document image is retrieved from storage in a first computer and electronically transmitted, by modem, for example, to a second computer for display on the second computer.
It has been conventional to scan a document combining black and white text with color image or grayscale image, or both, in a PC-based document management system using only a black and white (bi-level) scanner. Many disadvantages are attendant upon this approach, however.
First, scanning a color or grayscale image in black and white scanning mode not only loses all the hue information of a color original and the gradations in density of both color and grayscale images, but in many cases results in a mere conglomeration of black blobs. Text and line drawings scanned in a grayscale or color mode, on the other hand, become very blurry, and characters scanned in that fashion are not legible to optical character recognition processing ("OCR processing").
Moreover, even color scanning a grayscale image often produces unacceptable results. Although a color scanner should pick up the densities in a grayscale image well, inadequacies in the scanner may result in some "tint tainting" of the grayscale image data. That is, although the grayscale image is made up entirely of black, white and shades of gray and so has no chrominance or hue, the scanner may erroneously detect a slight hue in the grayscale image. This is because the color scanner cannot directly detect a gray value as such, but can only detect three predetermined primary colors, typically red, green and blue. When scanning an achromatic point, such as a point that is pure black, white or gray, the color scanner should detect exactly equal values for these three color components. In practice, however, slightly different values for the three color components may be detected, due to scanner inadequacies. Upon display or reproduction, the point will have a slight hue instead of being achromatic as it should be.
Thus, using one type of scanning for an entire document that includes color, grayscale or both, in addition to text, is not a viable approach.
Also, with document images (as opposed to text documents created locally in ASCII code using a word-processing program to begin with), it has been proposed to subject text portions of a document image to optical character recognition processing and to store the character information so obtained in ASCII form, greatly reducing the amount of storage required for the text portions. This technique, however, does not preserve any information regarding the type font used in the original document, and obviously is not applicable to non-textual portions of a document, or even to textual portions which are not in a font recognizable by the particular OCR process being employed.
The growing importance of desktop publishing in the business world only makes the problems described above more urgent. This technique has come to depend more and more heavily on scanning as a way to capture material, that is, of entering text, color images and grayscale images into a form usable in a desktop publishing system.